1. Field of the Invention
This invention relates to the return of reflux to a fractionating tower and is particularly concerned with a reflux return system for internally increasing the temperature of the reflux.
2. Description of the Prior Art
Distillation is the separation of the constituents of a liquid mixture by the partial vaporization of the mixture and separate recovery of the vapor and residue. A distillation carried out in such a manner that the vapor rising from a still comes in contact with a condensed portion of vapor previously evolved from the same still is called rectification. During rectification a transfer of material and interchange of heat result between the vapor and condensate, thereby producing a greater concentration of the more volatile components in the vapor than could otherwise be obtained with a single distillation operation using the same amount of heat. The condensed vapors returned to the rectification column are called reflux. Fractionation is a term synonymous with rectification and is commonly used to describe rectification carried out in the refining and petrochemical industries.
In a typical fractionation facility the overhead vapors from a fractionating tower are passed through a heat exchanger or condenser where they are condensed to form a liquid product, which is then passed to an overhead collection drum. The temperature of the liquid product collected in the overhead drum is normally well below the tower top temperature. A portion of this liquid product is returned to the column as reflux, normally by pumping it directly onto the reflux return tray. The sudden mixing of the cool reflux with the hotter liquid on the tray causes the temperature on that portion of the tray nearest the reflux inlet to drop rapidly.
For the purpose of heat economy it has been a common practice in the past to use a two-condenser, two-drum overhead system in lieu of a one-condenser, one-drum system in which the vapors are condensed, the resultant liquid is cooled to a relatively low temperature, and the liquid is collected in a single drum from which the reflux and product distillate are withdrawn. In the two-condenser, two-drum system, the overhead vapors from the fractionating tower are partially condensed in the first condenser and the resultant liquid is collected in the first drum at a temperature intermediate between the tower top temperature and the temperature of the product distillate. This liquid, which is at a temperature higher than could be obtained in a one-condenser, one-drum system, is then returned to the column as reflux. The remaining vapors in the first drum are passed through a second condenser where they are further cooled and condensed to form the distillate product, which is then collected in the second drum. In certain cases more than two condensers and drums have been used in fractionator overhead systems.
Although the use of a two-condenser, two-drum overhead system will allow the return of reflux to the fractionating tower at a higher temperature than can ordinarily be obtained using a one-condenser, one-drum system, the reflux will still be at a temperature significantly lower than that of the liquid on the reflux return tray and therefore the mixing of the reflux with the hot liquid on the tray will result in cool spots and a sudden decrease of temperature on or near the tray. In many instances this cooling will have little adverse effect on the operation of the fractionating tower. There may, however, be situations where a sudden drop and maldistribution of temperature on or near the reflux return tray will be deleterious to the fractionation process. Such will be the case if one of the components of the vapor is a substance that desublimes to form a solid when the temperature on or near the reflux return tray drops below a certain value. A gradual buildup of the solid material will plug the reflux return tray, decrease the efficiency of the fractionation and eventually force the shutdown of the fractionating tower. To insure that the temperature on the reflux return tray does not fall below the desublimation temperature, it may be necessary that the temperature of the cool reflux be substantially increased before it is passed onto the reflux return tray in the fractionating tower.